Project Summary Despite significant advances in the treatment of breast cancer, the metastatic form of the disease remains highly lethal, with a 5-year overall survival rate of only around 20%. Even with the use of new, targeted therapies such as antibodies against the human epidermal receptor 2 (HER2) expressed by cancer cells, over 70% of patients with metastatic diseases eventually become resistant to therapy. Advances in cancer immunotherapy with immune checkpoint blockers have shown that harnessing the power of the body's immune system can be an effective strategy to fight metastatic cancer. However, only a small proportion of patients (~20%) respond to immune checkpoint blockers, and their effectiveness against breast cancer remains uncertain. To overcome these challenges, we recently developed a multivalent bispecific nanobioconjugate engager (mBiNE) that simultaneously engages HER2 receptor on breast cancer cells and pro-phagocytosis molecules on macrophages to facilitate the clearance of HER2+ breast cancer by the immune system. The current proposal explores the use of mBiNE to promote antitumor immune responses and to enhance the effectiveness of immune checkpoint blockade against metastatic breast cancers. Aim 1 of the proposal will examine whether mBiNE activates potent and broad antigen-specific antitumor immune responses against HER2 overexpressing metastatic breast cancer. For Aim 2, we will elucidate the innate mechanisms within macrophages by which mBiNE-induced tumor cell phagocytosis promotes the cross-priming of adaptive antitumor immune responses. Finally, for Aim 3, we will investigate whether mBiNE can be safely combined with anti-PD1 therapy to stimulate both the innate and adaptive immune responses against poorly immunogenic metastatic breast cancer. An extensive training and development program has been proposed under the guidance of a multidisciplinary mentoring team consisting of physician-scientists, oncologists, immunologists, and tumor biologists. My primary mentor, Dr. Yang-Xin Fu, is a physician scientist, and a recognized expert in innate tumor immunity; my co-mentor, Dr. Wendy Woodward is known for her preclinical and clinical work with aggressive types of breast cancers. They will be joined on my mentoring committee by Dr. Irving Weissman, a pioneer in tumor immunology; Dr. Keith Knutson, a leader in breast cancer vaccine development; and Dr. Patrick Hwu, an internationally recognized cancer immunotherapy expert, who will serve as an advisor. The University of Texas Southwestern Medical Center provides an outstanding environment for my research and career development. All of the resources and equipment critical to the proposed research are readily available on campus, and I will have many opportunities to form collaborations with leaders in cancer immunotherapy. Together, the proposed research, training and career development plan will ensure that I receive the best mentorship available to become an independent investigator.